1. Field of the Invention
The invention relates to a colored light synthesis element and to a projection device using the colored light synthesis element.
2. Description of Related Art
Recently, a colored light synthesis element synthesizing red, green, and blue light and a projection device using the color synthesis element have been proposed. See, for example, Japanese Unexamined Patent Publications Nos. Hei 11-237604 (called “Patent Document 1” hereinbelow) and 2001-290010 (called “Patent Document 2” hereinbelow). A liquid crystal projection device described in Patent Document 1 and a projection type image display device described in Patent Document 2 separate white light from a lamp light source into three color components RGB by a dichroic mirror, and then perform a spatial modulation of light of the three color components by a liquid display element placed in each optical path. The light of the three color components is synthesized again by a colored light synthesis element. The synthesized light is projected by a projection lens, so that an image can be displayed.
Since white light is separated into three color components by a dichroic mirror, there is no overlapping of color components with respect to the light of the three color components. When the light of the three color components is synthesized using the difference in the color components by a colored light synthesis element having two dichroic surfaces, the light of the three color components can be efficiently synthesized.
Specifically, according to the liquid crystal projector device 100 described in Patent Document 1, as shown in FIG. 12, after white light from a high pressure mercury lamp 101 passes through an integrator (flyeye lens) 101a, the blue component is separated by a dichroic mirror 102 that transmits B component. Moreover, the green component and the red component are separated by a dichroic mirror 103 that transmits the R component. The light of each color component passes through LCD panels (a LCD 104 for R, a LCD 105 for G, and a LCD 106 for B) placed in each optical path to be synthesized again on two dichroic surfaces of a cross prism 107. The synthesized light is projected on a screen 109 through a projection lens 108. In this arrangement, the cross prism 107 is used as a colored light synthesis element, and includes two dichroic surfaces that are crossed, inside the element enclosed by three incidence surfaces and one emission surface.
According to a colored light synthesis element 110 used in the projection type image display device described in Patent Document 2, as shown in FIG. 13, the blue component and the green component are synthesized on a first dichroic surface 110a, and then the red component is synthesized on a second dichroic surface 110b. This structure is employed to avoid ghost caused by stray light produced between the opposite incidence surfaces of the cross prism, so that light from a single lamp light source is separated into light of individual color components. An object of resynthesizing the separated light is the same as that of the cross prism.
The emission spectrum for a high pressure mercury lamp generally used in a projection device that uses a conventional a colored light synthesis element is shown in FIG. 14. The line spectrum of mercury is fundamental. The spectrum around 440 nm is used as the blue component; the spectrum around 550 nm as the green component. The red component has no corresponding bright line spectrum, and uses weak continuous spectrum components. In addition, the line spectrum around 580 nm is the orange component. If the orange component is used as the red or green component, because brightness of color is lost, the orange component is not generally supposed to be used as unnecessary light component. Consequently, according to the conventional projection devices, since distance between the wavelengths of each color component is wide, separation or synthesis of color components was easily performed by dichroic mirrors or a dichroic prism.